May 10, 2012 -- People who have different
sleep patterns on the weekends than they do during the work week may experience "social jet lag," and a new study shows this shift in sleep schedule is linked to obesity.

For every hour of social jet lag, the risk of being overweight or obese rises about 33%, says researcher Till Roenneberg, PhD, a professor at the Institute of Medical Psychology at the University of Munich.

Roenneberg, who coined the term, says social jet lag is brought on by the shift in sleep schedule that many people experience on their days off, compared to work days. He estimates that it affects about two-thirds of the population.

It goes like this: You don't have to get up for work so you don't bother setting the alarm. That means you get up an hour or two later than you might during the work week. You may also push your bedtime back so you can go out with friends.

As a result, many people get more sleep on their days off than they do during the week, and they sleep on a slightly different schedule -- a schedule that is closer to their body's natural rhythms.

Roenneberg explains that switching sleep schedules this way feels like changing time zones.

"The behavior looks like if most people on a Friday evening fly from Paris to New York or Los Angeles to Tokyo and on Monday they fly back. Since this looks like almost a travel jet lag situation, we called it social jet lag," he says.

A key difference between travel jet lag and social jet lag, however, is light. When you arrive in a different place, the sun is coming up and setting at a different time, and your body can reset its own clock to match.

With social jet lag, the schedule disruption is chronic because a person stays in the same place.

"They have to live a life almost in a different time zone in comparison to their biological clock," Roenneberg says.

Social Jet Lag and Health

Roenneberg's previous research shows that social jet lag, although not usually as extreme as the sleep disruptions seen in shift workers, can still take a hefty toll on health.

"The more social jetlag you have, the more likely it is that you are a smoker; the more alcohol you drink; the higher your caffeine consumption -- you're slightly more depressed than the rest of the population," he says.

In his latest study, which is published in Current Biology, Roenneberg and his colleagues mined data from a large database, eventually gathering information on 65,000 people. He measured the average midpoint of a person's sleep on weekdays compared to weekends. The difference in those numbers gave him their social jet lag. For instance, Roenneberg's research shows that most of the population would naturally like to sleep between 1 a.m. and 9 a.m. These are the people most likely to experience social jet lag during the week, when they have to get up early for work. So, a person who goes to bed at 1 a.m. and gets up at 6 a.m. during the work week, for example, would have 1 1/2 hours of social jet lag, based on Roenneberg's formula.

Roenneberg then matched these hours of social jet lag to a person's self-reported body mass index (BMI) -- a measure of body size that takes into account height and weight.

He found that the more social jet lag a person experienced, the more likely they were to be overweight or obese.

Social Jet Lag and Weight Gain

So how could sleep that is out of sync with the body's clock potentially lead to weight gain?

One way, says Orfeu Buxton, PhD, an assistant professor of sleep medicine at Harvard Medical School, is that sleeping against the biological clock is closely linked to not getting enough sleep.

"To the extent that sleep duration and timing are up to us, our physiology is tuned to help us get enough," he says in an email to WebMD. "If our sleep is not up to us, we're much less likely to get enough to stay healthy. And whatever it is limiting our sleep may also limit time for exercise or preparing healthy meals."

In a recent study, Buxton showed that people like shift workers who have to sleep on short, disrupted schedules burned fewer calories at rest. He estimated that the drop in metabolism could lead to a gain of 10 to 12 pounds in a year's time.

So how can you counter the effects of social jet lag? It's not so easy. When asked, Roenneberg said trying to keep your work schedule on weekends might not really help because people keep those kinds of abnormal schedules (getting up at 6 a.m.) because they have to, not because they really want to. Trying to keep the same disrupted schedule on the weekend would just add to a person's sleep debt, which isn't a great solution.

"The consequences of sleep debt are enormous," he says.

In a perfect world, Roenneberg says, society's whole attitude toward sleep would change. For example, work schedules would be more aligned with the biological clock and start later so that people could go to sleep later. Sleep, he thinks, should get more respect.

Still, daylight may help, Roenneberg says. Early risers who feel like they need help falling asleep earlier at night -- so that they can get more sleep -- may be helped by getting more sunlight in the morning and avoiding sunlight in the afternoon and evening.

People who would like to stay up a little later should try to get more sunlight in the afternoon and evening, he says.